JPH04109193A - Fuel assembly - Google Patents

Abstract

PURPOSE:To reduce the kind of Pu enrichment levels and increase the Pu loaded quantity for each assembly by setting the average enrichment level of plutonium (Pu) at corner sections on the control rod side lower than that at other corner sections, and increasing the number of fuel rods containing an inflammable poisonous substance. CONSTITUTION:A fuel assembly is divided into the first region (wide gap side) on the corner section A side and the second region (narrow gap side) on the corner section D side across the diagonal line connecting two opposite corner sections B, C not on the control rod 11 side. In the second region, fuel rods 14 with the medium enrichment level 3 are arranged near the corner sections B-D on the outermost peripheral portion, those with the high level 2 at the other positions on the outermost peripheral portion, and those with the highest level 1 at positions other than on the outermost periphery, where fuel rods 14 with encircled numbers 1-4 are MOX fuel rods and the smaller number indicates the higher Pu enrichment level. In the first region, fuel rods 14 with the low enrichment level 4 are arranged near the corner section A on the outermost periphery, those with the medium level 3 at other positions on the outermost periphery, those with the high level 2 on the second and third layers, and fuel rods G containing gadolinium are mostly arranged in the first region. The kind of the Pu enrichment levels is reduced, the Pu loaded quantity for each assembly can be increased, and uranium can be saved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a fuel assembly, which is particularly used in a boiling water reactor, and which is a fuel assembly that is used for a uranium-plutonium mixed oxide fuel (hereinafter referred to as M
This relates to a fuel assembly filled with OX fuel (referred to as OX fuel).

[Conventional technology]

The structure of a fuel assembly used in a boiling water reactor is shown in Figure 5. 4 is an upper tie plate, 5 is a channel fastener, 6 is a channel box, 8 is a spacer, 9 is a lower tie plate, and 10 is a fuel rod. It is.

As shown in FIG. 6, the core called the D lattice of a boiling water reactor has different gap intervals between adjacent fuel assemblies, and the gap interval on the side where the control rods 12 are inserted (control rod side) is different. (Dz in Fig. 6, hereinafter referred to as wide gap) is the gap interval on the side where the control rod 11 is not inserted (side without control rod) (Dz in Fig. 6, hereinafter referred to as narrow gap)
Therefore, on the wide gap side, the area of the moderator flow path outside the channel box 13 is large, and the effect of moderating neutrons is large. For this reason, the number of eight fuel rods 14 in the fuel assembly 12 tends to be higher on the wide gap side and lower on the narrow gap side.

Examples of plutonium enrichment distribution and uranium enrichment distribution of a fuel assembly containing conventional MOX fuel (hereinafter referred to as an MOX fuel assembly) are shown in FIGS. 3 and 4.

FIG. 3 shows an example of plutonium enrichment distribution and uranium enrichment distribution of a discrete MOX fuel assembly. In FIG. 3, 11 indicates a control rod, and 12
indicates a MOX fuel assembly.

The fuel assembly 12 is arranged in a channel box 13 . fuel rod 1
4 and a water rod 15.

In the fuel rod 14, letters P1 to P6 indicate fuel # in which uranium contains plutonium oxide (hereinafter referred to as MOX fuel rod), and letter G indicates a uranium fuel rod (containing uranium in the state of a new fuel assembly). fuel rods that do not contain plutonium)
shows a fuel rod containing gadolinia. In addition,
Regarding MOX fuel rods, the smaller the number of the subscript of letters PI to P8, the higher the plutonium enrichment.

FIG. 4 shows an example of plutonium enrichment distribution and uranium enrichment distribution of an island-type MOX fuel assembly. In the fuel rod 14 of FIG.

Numbers 1 to 4 indicate uranium fuel rods, and the smaller the number, the higher the enrichment of uranium.

Further, the letters Ps and Px indicate MOX fuel, and the letter G indicates a fuel rod containing gadolinia in a uranium fuel rod.

Although discrete MOX fuel assemblies can carry a large amount of plutonium per unit, they require a large number of enrichment types. In addition, island-type MOX fuel assemblies can reduce the number of enrichment types in contrast to discrete-type MOX fuel assemblies, but the amount of plutonium loaded per fuel assembly is reduced.

Furthermore, since MOX fuel assemblies generally have a harder neutron spectrum than U 2 O 2 fuel assemblies, the value of gadolinia, which is a burnable poison, is quite different from that of U 2 O 2 fuel assemblies. In other words, the neutron absorption cross section of gadolinia is large for thermal group neutron energies and small for neutron energies larger than this, so as the neutron spectrum becomes harder, the reactivity value of gadolinia decreases, and at the same time the burning rate of gadolinia slows down. . For this reason, in the MOX fuel assembly, the gadolinia concentration is lowered and the number of gadolinia-containing fuel rods is increased compared to the U○2 fuel assembly (which does not contain plutonium in the state of a new fuel assembly).

[Problem to be solved by the invention]

An object of the present invention is to provide a fuel assembly that can reduce the types of plutonium enrichment and increase the amount of plutonium loaded per assembly.

[Means to solve the problem]

In order to achieve the above object, the present invention divides a cross section of a fuel assembly with a diagonal line connecting two opposite corners where a channel fastener is not located as a boundary between a first region including a corner portion where a channel fastener is located and a channel. When divided into a second region that does not include the corner where the fastener is located, the average enrichment of plutonium in the first region is
The number of fuel rods containing burnable poison is greater in the first region than in the second region.

[Effect]

As mentioned above, the average plutonium enrichment in the first region of the MOX fuel assembly is lower than that in the second region, and more fuel rods containing burnable poison are arranged in the first region than in the second region. This affects the fuel properties in the following way.

Since the first region of the fuel assembly is adjacent to the wide gap, the area of the moderator flow path outside the channel box is large, and the neutron moderating effect becomes large. Therefore, the local power peaking will be higher in the first region and the second region facing the narrow gap will be higher.
lower in the area. In order to keep local power peaking low, while preserving the fuel assembly average plutonium enrichment, lower the average plutonium enrichment in the first region and increase the plutonium enrichment in the second region accordingly. According to
The output peaking in the first region is low and the output peaking in the second region is high, and the output distribution can be made flat.

Next, by utilizing the difference in average enrichment of plutonium between the first and second regions and arranging many fuel rods containing burnable poison in the first region, we will increase the reactivity value of gadolinia, which is a burnable poison. The decline can be suppressed. That is, by lowering the average enrichment of plutonium in the first region, which has a large neutron moderation effect, the hardening of the neutron spectrum in the first region is alleviated, and the hardening of the neutron spectrum in the second region is increased. Placing fuel rods containing gadolinia in the second region will reduce the reactivity value of gadolinia, which will lead to an increase in the number of fuel rods containing gadolinia, which will have the opposite effect because it will reduce the amount of plutonium loaded per assembly. It becomes an effect.

If fuel rods containing gadolinia are placed in the first region, the reactivity value of gadolinia does not decrease, so the number of fuel rods containing gadolinia can be reduced, and the amount of plutonium loaded per assembly can be increased.

Also, to flatten the power beak of the outermost fuel rods adjacent to the wide gap in the first region.

Although there were many types of plutonium enrichment.

By arranging fuel rods containing gadolinia in the first region, the power distribution in the first region is suppressed, and the power peaking of the outermost fuel rods adjacent to the wide gap in the first region becomes flat, so that plutonium enrichment The number of degrees can be reduced.

〔Example〕

A fuel assembly which is an embodiment of the present invention will be explained with reference to FIG. In FIG. 1, 11 represents a control rod, and 12 represents a MOX fuel assembly. The fuel assembly 12 is connected to a channel box 13. It is composed of fuel rods 14 and water rods 15. The appearance of the fuel assembly 12 is the same as that shown in FIG. The fuel assembly 12 is a new fuel assembly with a burnup of 0 GWd/l. The fuel rods 14 are loaded with either pellets of MOX fuel or pellets of uranium oxide containing no plutonium mixed with gadolinia. Among the fuel rods 14, the fuel rods are marked with numbers 1 to 4.

These are MOX fuel rods. The smaller the number, the higher the plutonium enrichment. Furthermore, fuel rods marked with the letter G indicate fuel rods containing gadolinia filled with the above-mentioned pellets containing gadolinia in uranium oxide.

When the fuel assembly 12 is loaded into the reactor core, the channel fastener 5 is located on the control rod 11 side, that is, on the wide gap side.

In FIG. 1, the channel fastener 5 is located at the corner A and is attached to the tie plate 4. A cross section of the fuel assembly 12 is divided into a first area on the corner A side and a second area on the corner side from the diagonal line connecting the two opposing corner parts B and C where the channel fastener 5 is not located. Divide into regions. The channel fastener 5 is not located in the corner portion included in the second region.

In the plutonium enrichment distribution of the fuel assembly 12, in the second region (narrow gap side region), the highest enrichment fuel rod is 1 except for the outermost fuel rod, and the outermost corner and the vicinity are the middle enrichment fuel rods. The rod 3 and the other outermost portions are high enrichment fuel rods 2. In the first region (wide gap side region), the outermost fuel rods are the lowest enrichment fuel rods 4 near the corner A and the other medium enrichment fuel rods 3. 2nd layer.

The fuel rods in the third layer are high enrichment fuel rods 2.

Due to this plutonium enrichment distribution, the average enrichment of plutonium differs between the wide gap region and the narrow gap region. Many of the fuel rods G containing gadolinia are arranged in the wide gap side region.

Next, FIG. 2, which is another embodiment of the present invention, is shown.

The example shown in Fig. 2 is an example in which the number of gadolinia-filled fuel rods G in Fig. 1 is increased from 8 to 10, and the average plutonium enrichment in the assembly is the same as in Fig. 1 on the wide gap side. There is a difference between the area and the narrow gap side area. In this way, if there are many zero fuel rods containing gadolinia,
As many fuel rods containing gadolinia as possible are placed in the wide gap side area, and the rest are placed near the water rods in the narrow gap side area.

In all of these embodiments, all fuel rods except for the fuel rods in which gadolinia is mixed with uranium oxide are fueled with MOX fuel, so it is possible to increase the amount of plutonium loaded per MOX fuel assembly. Furthermore, the types of plutonium enrichment in the MOX fuel assembly can be reduced from six types in the conventional discrete '1tox fuel assembly to four types. When using plutonium as fuel,
Because fuel pellet molding is carried out in a completely sealed container (glove box), cleaning the glove box when changing the plutonium enrichment takes more time than in the case of uranium, and the operating rate during production decreases significantly. Become. Therefore, reducing the types of plutonium enrichment used in MOX fuel assemblies has the effect of reducing the manufacturing time of MOX fuel assemblies.

Note that the present invention is not limited to the above embodiments.

For example, the MOX fuel rods with the lowest enrichment in the corners of the wide gap region may be replaced with uranium fuel rods, and the number of enrichment types of MOX fuel rods may be three.

Even in this case, the amount of plutonium loaded per MOX fuel assembly can be kept at a maximum, and the cost of molding the MOX fuel can be reduced.

Furthermore, if the fuel rod arrangement of normal uranium fuel assemblies is changed from the 8 rows and 8 columns shown so far to 9 rows and 9 columns, the MOX fuel assemblies should be arranged in 9 rows and 9 columns. However, it is possible to implement the present invention.

〔Effect of the invention〕

The fuel assembly of the present invention can reduce the types of plutonium enrichment and increase the amount of plutonium loaded per fuel assembly, so it can shorten the manufacturing time of the fuel assembly and save uranium resources. can.

[Brief explanation of the drawing]

1 and 2 are cross-sectional views of a MOX fuel assembly which is an embodiment of the present invention, FIGS. 3 and 4 are cross-sectional views of a conventional MOX fuel assembly, and FIG. 5 is a cross-sectional view of a conventional MOX fuel assembly. FIG. 6 shows a cross-sectional view of the fuel assembly. 4... Upper tie plate, 5... Channel fastener, 8... Spacer, 9... Lower tie plate, 1
DESCRIPTION OF SYMBOLS 1... Control rod, 12... Fuel assembly, 13... Channel box, 14... Fuel rod, 15... Water rod.

Claims (1)

[Claims] 1. an upper tie plate, a lower tie plate, a plurality of fuel rods whose ends are held by the upper tie plate and the lower tie plate, and a channel fastener attached to one corner of the upper tie plate; and in which some of the fuel rods are fuel rods containing a burnable poison, the cross section of the fuel assembly is defined by a diagonal line connecting two opposing corners where the channel fastener is not located. When divided into a first region including the corner portion where the channel fastener is located and a second region not including the corner portion where the channel fastener is located, the average plutonium enrichment of the first region is equal to , and the number of fuel rods containing the burnable poison is greater in the first region than in the second region.